Thanks for recommendations!
I've just contacted the Airbnb support, waiting for their response. Since it's the first time I'm experiencing this type of issue, I haven't thought about asking the support to contact my guest.
Yes, I also always ask guests to set their check-out time right when they arrive, if they are unsure we usually agree on contacting each other the day before, but I always insist on letting me know the default time at once, then I just reach out to them to make sure that there are no changes.
It is late night here already, but I hope everything is good and he'll respond tomorrow, or the support team will reach out to him by that time. If there are no updates I think it'll be better for me to come earlier than twelve o'clock.